Maybe you have used this gmail account earlier and it’s blacklisted in Gmail OR you have mentioned sender’s name and email that are different from ones that are mentioned in the Gmail account you are using. Gmail is suspicious to those who use different name and sender’s address for the external Gmail smtp. As you know you can easily change sender’s name and email address and it’s not connected with the external Gmail SMTP and the related account. So try using the name and the email address you created for gmail profile.